Power monitoring systems monitor the flow of electric power in circuits through a plant or other facility. In the POWERLOGIC® system manufactured by Square D Corporation, circuit monitors and power meters are dedicated to power monitoring, while other compatible devices collect additional equipment information from protective relays, circuit breakers, transformer temperature controllers, and panelboards. Electrical data, such as current, power, energy, waveforms, and equipment status, is passed over a data network to one or more personal computers. The personal computers run power monitoring application software that retrieves, stores, organizes, and displays real-time circuit information in simple, usable formats. The information collected and stored in a power monitoring system helps operate a facility more efficiently. The quality of the data depends upon the accuracy of the instrumentation and the usability of the display formats.
The power meter can replace conventional metering devices such as ammeters, voltmeters, and watt-hour meters while providing other capabilities not offered by analog metering. The power meter's true rms readings reflect non-linear circuit loading more than conventional analog metering devices. The power meter can perform residual measurements such as calculating the neutral current even when the user does not provide a neutral current transformer. The power meter assists in identifying overloaded neutrals due to either unbalanced single phase loads or triplen harmonics. Circuits can be closely monitored for available capacity by keeping track of the peak average demand current.
Power quality standards for industrial power meters and circuit monitors require anti-aliasing circuitry for data integrity. The specifications in these standards are very concise. For example, power quality standards such as IEC 61000-4-7 2nd edition require that power signals be anti-aliased at least 50 dB at the Nyquist frequency. However, with fundamental frequencies that vary depending on the geographic region from 45 to 66 Hz, it is very difficult to design a filter that can be easily and quickly reconfigured especially when harmonic content up to the 50th or above is of concern.
There are solutions to this problem such as, e.g., utilizing a switched capacitive filter coupled to an analog-to-digital filter. However, there is a fundamental problem with using switched capacitive filters. Specifically, if the clock signal driving the filter is not synchronous with an analog-to-digital (A/D) converter's master clock, coupled thereto, a beat frequency between the two asynchronous clocks will result and cause a measurement error in the signal. This error results from the internal sample and holds of the A/D converter latching the signal while the switched capacitive filter is changing it. The A/D converter's internal sample and holds are synchronous with its master clock. This would also result if one used external sample and holds with their analog to digital converter and the clocks were not synchronous.
Previous designs suffer from defects. For example, prior systems that utilize continuous time filters have fixed corner frequencies. Accordingly, the harmonic accuracies of such designs vary based upon the fundamental frequency of the detected signal. Also, such devices use only a pre-selected bandwidth for performing the monitoring, thereby preventing the devices from detecting certain high harmonics of the fundamental (e.g., harmonics above the 100th order). The present invention is directed to satisfying this and other needs.